1. Field of the Invention
The present invention relates to a lithographic pellicle, in particular to a lithographic pellicle used as dust-proof protection in the manufacture of semiconductor devices such as LSI or ultra-LSI. More particularly, the invention relates to a lithographic pellicle frame used for ultraviolet exposure light of 200 nm or shorter wavelength used for patterning light exposure which requires high resolution.
2. Description of the Related Art
Conventionally, the manufacture of semiconductor devices such as LSI and ultra-LSI, or liquid crystal display panels and the like, has involved employing procedures such as lithography for the patterning of semiconductor wafers or liquid crystal original plates through irradiation of light. However, there is a problem that any dust adhering to the employed original plate absorbs and reflects light, which deforms and roughens the edges of the replicated patterning, thereby detracting from dimensions, quality, and appearance, and impairing the performance of the semiconductor device and/or liquid crystal display panel, while reducing the manufacturing yield thereof.
Thus, these operations are ordinarily carried out in clean rooms, but keeping exposure original plates clean at all times in such clean rooms is difficult, and hence pellicles having good light transmissivity are adhered, as dust-proof protection, to the surface of exposure original plates. The advantage of the pellicle is that dust does not attach directly to the surface of the exposure original plate, but becomes adhered to the pellicle membrane, so that if focus is in accord with the pattern of the exposure original plate during lithography, transfer is not affected by dust on the pellicle.
The pellicle is made up of a pellicle frame comprising aluminum, stainless steel, polyethylene or the like, a transparent pellicle membrane adhered on the upper surface of the pellicle frame, comprising nitrocellulose, cellulose acetate or the like having good light transmissivity, an adhesive layer coated on the lower surface of the pellicle frame, and a release layer (separator) adhered on the adhesive layer. The adhesive bonding between the pellicle frame and pellicle membrane is carried out by coating a good solvent for the pellicle membrane material and then air-drying the solvent (Japanese Patent Application Laid-open No. S58-219023) or using an adhesive agent such as an acrylic resin, epoxy resin or the like (U.S. Pat. No. 4,861,402, Japanese Patent Examined Application Publication No. S63-27707, Japanese Unexamined Patent Application Laid-open No. H07-168345).
As a result of ever higher lithography resolutions encountered in recent years, the employed light sources are gradually shifting to shorter wavelengths in order to realize such resolutions. Specifically, there has been a shift towards g-line (436 nm), i-line (365 nm), KrF excimer lasers (248 nm) in ultraviolet light, while ArF excimer lasers (193 nm) have begun to be used recently.
In a semiconductor exposure device, the pattern drawn on a photomask is burned onto a silicon wafer by way of short-wavelength light. Irregularities on the photomask and the silicon wafer give rise however to focal shift, which impairs the pattern printed onto the wafer. The required flatness from photomasks and silicon wafers is getting more stringent as the patterning becomes finer and finer. For instance, the required flatness from photomasks is becoming gradually more demanding, from a flatness of 2 μm at the pattern plane, down to 0.5 μm and 0.25 μm for the 65 nm node and beyond.
A pellicle is affixed to a photomask to protect a pattern from dirt after the photomask is completed, but affixing the pellicle to the photomask may cause a change in the flatness of the photomask. This is generally thought to be an effect of unevenness of the pellicle frame on the planarity of the photomask. Furthermore, the change of the flatness due to the pellicle affixation unfortunately may also cause a deformation of the circuit pattern made on the photomask. This is thought to be due to a change of the relative position of the pattern on the photomask due to the change in the planarity of the photomask.
The pellicle is affixed to the photomask by a photomask adhesive applied to one side of the pellicle frame, but in the case where the pellicle is affixed to the photomask, normally, the pellicle is attached by pressure to the photomask with a force of about 20 to 30 kgf. Conventional pellicle frames generally are made of aluminum alloy. Pellicle frames used in semiconductor lithography have a width of about 150 mm and a length of about 110 to 130 mm, and have a shape with an opening in a central region. Generally, pellicle frames are manufactured by cutting a plate of aluminum alloy into the pellicle frame shape, or extrusion molding of aluminum alloy material into the pellicle frame shape.
Generally, the planarity of a photomask exhibits a TIR value of several μm or less, and the same of a leading-edge photomask is 1 μm or less; while the planarity of the pellicle frame, generally being about several tens μm, is comparatively larger than that of the photomask. Therefore, in the case where the pellicle frame is affixed to the photomask, the unevenness of the pellicle frame may cause a change in the flatness of the photomask. Here, it is thought that improving the flatness of the pellicle frame to the level of the flatness of the photomask will allow a reduction of the planarity change of the photomask. However, the pellicle frame has a thin frame shape with a width of about 2 mm, and therefore is easily deformed; and it is not always easy to make a flat pellicle frame. Therefore, it is difficult to achieve a flatness of the pellicle frame similar to the level of that of the photomask.
Moreover, even in the case where a pellicle frame having a very good flatness is used, in the case where the flatness of a surface of the adhesive layer is poor, the flatness of the adhesive affects the flatness change of the photomask. Normally, during the affixation of the pellicle, pressurizing the pellicle causes firstly convex portions of the adhesive layer to contact the photomask, and then finally concave portions to contact; and the affixation is completed without an air path anywhere around the entire periphery of the pellicle. However, in this case, the initially contacting convex portions of the adhesive undergo more deformation from the affixation, and therefore the deformation stress in these portions is larger than that of the concave portions, thereby resulting in an uneven distribution of stress in the surface. As a result, the affixation of the pellicle causes a deformation of the photomask.
In consideration of circumstances such as those recited above, the present invention is directed to prevent the occurrence of distortion of the photomask even in the case where the pellicle is affixed to the photomask.